Elements of a construction or assembly set, and accessories

ABSTRACT

Detachable links are formed by a first link having a part-spherical end joined to a cylindrical first intermediate portion and a second link having a receiving end with spaced, generally U-shaped arms defining a slot. The arms resiliently spread apart to receive and then snap back to capture the part-spherical portion, leaving the cylindrical portion in the slot. The joined links may be freely relatively pivoted through substantially 180 degrees and may be freely relatively rotated through a full 360 degrees about the axis of the cylindrical portion. The part-spherical portion may have one upstanding cylindrical stub or a diametrically opposed pair which are positionable either between the legs of the arms or in the slot between arms. These stubs limit the free rotation between links by resisting same except as permitted by forcible rotation in 90 degree steps due to spreading apart and snapping back of the arms. In one position where the stubs are located between the legs of the arms, free pivoting through 180 degrees as the cylindrical portion traverses the slot is permitted but forcible stepwise axial rotation is still required. When the stubs are in the slot pivoting is limited except as permitted by forcible pivoting in 90 degree steps. If only one stub is present, free pivoting through 90 degrees is permitted for about 90 degrees of the slot while forced pivoting through the remaining 90 degree traverse is required.

The invention relates to construction or assembly elements provided withcoupling members serving for associating these elements with oneanother, and more particularly a set of construction elements suited toone another, by means of which it is possible to produce a very largenumber of combinations serving as toys.

For this purpose, the coupling members for the construction elements orlinks of the present invention consist of at least one spherical part,on the one hand, constituting a penetrating element, and at least onereceptacle corresponding to the spherical parts, on the other hand,which are designed in such a way that a spherical part of one link canbe locked elastically in the receptacle of another element forming thereceiving link.

According to a preferred embodiment of the invention, the penetratinglink has at least two identical spherical parts which are connected toone another by means of an intermediate cylindrical narrowed portion,the distance between the centres of these spherical parts beingsubstantially equal to the diameter of the latter.

The receiving link can be formed by a single or multiple link which hasat its two opposite ends receptacles which are intended for receivingspherical parts such as those mentioned above.

The abovementioned receptacle can likewise be limited on at least one ofits sides by an elastically articulated lip which grips the sphericalpart of the coupling member located in the receptacle.

Other advantages and special features of the invention will emerge fromthe description given below of some specific embodiments of theconstruction elements according to the invention.

FIG. 1 is an embodiment of a penetrating link;

FIGS. 2 and 3 are perspective representations of special forms of thepenetrating links;

FIGS. 4, 5 and 6 show respectively in perspective a receiving link, amodified penetrating link and their assembly, FIG. 6 including a partialcutaway portion;

FIG. 7 is a graphical representation of the relative dimensions of theelements according to FIGS. 4, 5 and 6;

FIG. 8a is a top view of a receiving link;

FIG. 8b is a front view of a receiving link;

FIG. 8c is a side view of a receiving link;

FIG. 9a is a top view of a penetrating link;

FIG. 9b is a side view of a penetrating link; and

FIG. 10 is an alternative embodiment.

In the various figures, the same reference numerals denote identical orsimilar elements.

The penetrating link 2 illustrated in FIG. 1 is provided with twospherical parts 7 which are fixed to one another by means of acylindrical intermediate piece 17 in such a way that a cylindrical outersurface is formed between these parts 7. The distance "a" between thecentres of these spherical parts is approximately equal to the diameterof the latter.

FIG. 2 shows three spherical parts fixed to one another by intermediatepieces 17, whilst FIG. 3 shows a penetrating link 2 consisting of acubic box 19 to which, for example, two spherical parts are fixed atright angles to one another, whilst in the two open ends of the box itis possible to mount two spherical parts 7 removably by means of aconnector 20 which can be slipped into this box 19.

This connector has ribs 22 on its side faces which interact withcorresponding notches 21 which are made in the inner walls of the boxand which therefore make it possible to retain the connectorsufficiently firmly in the box in a removable manner.

The length of the connector 20 is at most equal to half the length ofthe box 19, so that it is possible to fix to it, at the same time, aspherical part 7 in both ends.

It is therefore possible, on this special element, to build in differentdirections at right angles to one another.

In the embodiment shown in FIGS. 4, 5 and 6, a receiving link 1 isformed by a link which has at its two opposite ends receptacles 5 forthe spherical parts 7 of a penetrating link 2.

This link 1 consists of two H-shaped parts 9 and 10 parallel to oneanother and spaced from one another at a distance which is a little lessthan the diameter of a spherical part 7, so that the latter can belocked between the branches or legs 11 and 12 of these H-shaped parts 9and 10.

For this purpose, the branches or legs 11 and 12 of the H-shaped part 9are connected elastically to the corresponding branches or legs 11 and12 of the H-shaped part 10 opposite to them, so as to form receptacles 5for the spherical parts 7.

In the embodiment according to FIGS. 4 and 6, only the free ends of thebranches 11 and branches 12 of the two H-shaped parts 9 and 10 areconnected two by two by curved intermediate pieces or bights 13 whichare located in the plane of the corresponding branches. It will be notedthat portions 11, 13, 11 form generally U-shaped arms extending fromopposite ends of link 1, and portions 12, 13, 12 form generally U-shapedarms extending from opposite ends of link 1, these latter arms beingparallel with the first mentioned arms.

To make it easier for the penetrating link 2 to penetrate, notches 6 canbe provided in the curved pieces 13, as shown in FIGS. 4 and 6.

It will also be noted that the surfaces delimited respectively by thebranches 11 and the upper curved parts 13, on the one hand, and by thebranches 12 and the lower curved parts 13, on the other hand, can besolid and can serve as an advertising medium or the like.

Advantageously, at least some of the spherical parts 7 are provided withlocking members which make it possible to limit the rotary movement ofthe spherical part in question in a receptacle 5 of the receivingelement.

In the embodiment according to FIGS. 5 and 6, a locking member of thistype is formed by a circular shoulder 14, the centre of which is locatedon an axis perpendicular to that on which the centres of the twospherical parts 7 of the penetrating element 2 are located.

The height of the shoulder 14 is at most limited by the plane tangent tothe spherical part 7 at the centre of this shoulder; it is thereforepossible for the latter to be located lower down.

Furthermore, the penetrating link according to FIGS. 5 and 6 has acylindrical cavity 15, the axis of which is located on the axisconnecting the centres of the two spherical parts, this cavity openingat its two ends onto the outer surface of these spherical parts 7.

In some embodiments, the centre of this shoulder can be located on thesame axis as that connecting the centres of the two spherical parts.

In other cases, shoulders diametrically opposite one another can beprovided on one and the same spherical part 7.

In yet other cases, only one shoulder need be provided on one of thespherical parts.

The centres of the various shoulders can also be located on one and thesame axial plane of the penetrating element, or the centres of theshoulders of one of the spherical parts can be located in an axial planeperpendicular to the axial plane containing the centres of the shouldersof the other spherical part.

In the embodiment according to FIGS. 5 and 6, two shoulders, the centresof which are diametrically opposite one another and are located in oneand the same axial plane, have been provided on each of the sphericalparts.

The rotary movement of the penetrating link 2 in relation to thereceiving element 1 is limited as a function of the position of thelocking member, more specifically of the shoulder 14, on the sphericalpart.

Thus, for example, according to FIG. 6, the penetrating element 2 canexperience rotation only along a single axis parallel to the webs 16 ofthe H-shaped parts 9 and 10. If the centre of the shoulder is located onthe axis connecting the centres of the spherical parts, that is to sayon the same axis as the cavity 15, the penetrating element 2 can thenrotate only about the axis of this cylindrical cavity.

It is clear that what has just been described in relation to theshoulders can apply to all embodiments of the penetrating link,particularly to the embodiments shown in FIGS. 2, 3 and 5.

In order to reduce to a minimum the free play between the variousconstruction elements connected to one another and therefore to obtainan exact linking-up of these elements, together with a maximum number ofcombination possibilities, it is important that the relative dimensionsof the receiving elements and penetrating elements should satisfyspecific conditions.

These dimensions are shown graphically in FIGS. 7, 8 and 9.

Thus, it is important that the external distance a between the H-shapedparts should be approximately equal to the diameter of the sphericalparts 7 and that the internal distance b between the latter should beapproximately equal to the diameter of the narrowed portion 17 betweenthe two consecutive spherical parts 7. Furthermore, the width of theH-shaped parts is approximately equal to the diameter of the sphericalparts 7 and constitutes two receptacles 5 opposite to one another forreceiving the spherical parts, the H-shaped parts being produced in sucha way that the centres of the spherical parts are located in thesereceptacles spaced from one another at a distance equal to theirdiameter.

The space 18 between the web 16 and the base of the branches which areconnected on one side to this web has the shape of a semi-circle, theradius of which corresponds approximately to that of the circularshoulder 14. The centre O of this semi-circle is located in thelongitudinal plane of symmetry S₁ of the H-shaped parts 9 and 10perpendicular to the web 16 and at a distance from the centre of thelatter which is approximately equal to the radius of the sphericalparts.

Said curved intermediate pieces 13 also have approximately the shape ofa semi-circle, the radius of which is approximately equal to that of thecircular shoulder 14 and the center O' of which is located in the middlebetween the two H-shaped parts 9 and 10 at a distance from thetransverse plane of symmetry S₂ which is approximately equal to theradius of the spherical parts. The centres O and O' are thereforelocated in one and the same plane, parallel to the plane of symmetry S₂.

The distance x, which is one-half the quantity a minus b conformsapproximately to the following equation: ##EQU1## in which x is thedepth of the narrowed portion and R is the radius of the spherical parts7. This formula is illustrated graphically in FIG. 7. By inspection, thetriangle illustrated is an isosceles triangle in which its two sides areR-x or the radius of the cylindrical portion 17, so that R-x/R=1/2.

On the basis of this formula it is possible to determine all thedimensions both of the receiving element 1 according to FIG. 4 and ofthe penetrating link 2 according to FIG. 5. Thus the branches 11 and 12of the H-shaped parts have a square cross-section, the side of which isequal to x.

As regards FIG. 4, it is clear that if an excess thickness is providedto close, for example, the upper surface delimited by the curved part 13and the branches 11, the dimensions must take this excess thickness intoaccount.

The receptacles 5 have a penetration aperture or slot 30 for thespherical parts 7, of which the smallest width corresponding to theinternal distance between the intermediate pieces 13 is equal to thediameter of the spherical parts less twice the depth x of the narrowedportion 17 mentioned.

The foregoing description made with reference to FIGS. 4 to 9 appliesaccordingly to the embodiments of FIGS. 2 and 3 which relate topenetrating elements 7 serving to form T-shaped or angular connectionswith receiving elements 1 (not shown).

It goes without saying that it is possible to envisage yet other formsof penetrating elements or receiving elements.

Thus, for example, the links can be single, as in FIG. 4, or multiple.As regards multiple links, these can consist, for example, of two ormore single links joined to one another.

The link elements 1 of FIG. 4 can be assembled into an open or closedchain or into a branched construction by alternately connecting apenetrating element and a receiving element which are connected to oneanother.

Depending on the type of spherical parts 7 and their relative positionin the receptacles 5, it is possible to obtain a completely rigidassembly, a partially rigid assembly or a completely flexible assemblyand to limit the relative angular movements and the relative position ofthe circular shoulder 14 in relation to the receptacle 5. That is tosay, if two links are joined as in FIG. 6, with a stub or shoulder 14 ordiametrically opposed stubs positioned between the legs 11, 12 of thearms, against the bights 13, these two links are free to pivot in theplane to which the arms are parallel with the portion 17 traversingthrough the slot 30 between the arms. In this case, the two links arefree to pivot through an angle of approximately 180 degrees as thecylinder traverses the slot, just as when the penetrating link has nostubs as in FIG. 1, but relative rotation therebetween about the axis ofthe cylinder 17 requires forcible rotation in 90 degree steps instead ofthe free rotation as presented by the FIG. 1 form. If there is but onestub 14 on a part-spherical portion 7 and the two links are joined sothat this stub is positioned in the slot between the arms (e.g., rotated90 degrees from the position shown in FIG. 6), then the two links arestill free to pivot in the aforesaid plane but only through an angle ofabout 90 degrees while the remaining 90 degrees of traverse of thecylinder in the slot requires a forcible pivoting. Again, forciblerotation about the axis of the cylinder is required. Lastly, if thereare two stubs 14 on the part-spherical portion 7 and the two links arejoined so that the two stubs or shoulders 14 are positioned in thearcuate ends of the slots, neither free pivoting nor free rotation ispossible between these two links and the forcible pivoting and forciblerotation, as above, is required.

It is appropriate to note that many alternative embodiments of theinvention arising from the principle claimed are possible. Thus, it ispossible to conceive a link such as that shown in FIG. 10, whichcomprises a single receptacle 5. Links of this type can be linked up bymaking a spherical part 7 of a first element penetrate into thereceptacle 5 of an adjacent link (not shown). However, this alternativeembodiment is more difficult to produce by the injection of resins.

I claim:
 1. A detachable link type of device which comprises:a firstlink having opposite end portions connected by a first intermediateportion and a second link having opposite end portions connected by asecond intermediate portion, one end portion of the first link having aspherical surface portion and the first intermediate portion being inthe form of a cylinder joined to said one end portion to define acylindrical, annular surface immediately adjacent said one end portion;one end portion of said second link being adapted to receive and capturesaid one end portion of the first link in snap-fit relation, said oneend portion of the second link including a pair of generally U-shapedand parallel arms extending from said second intermediate portion andwhich are spaced apart a distance less than the diameter of saidspherical surface portion to define a slot therebetween, each of saidarms including a bight, said slot being of a width substantially equalto the diameter of said cylinder and said arms being resilientlydeformable to spread apart and pass said spherical surface portiontherebetween and then snap back toward said cylinder to capture saidspherical surface portion of the first link, said slot extending fromsaid intermediate portion on one side of said second link to theintermediate portion on the other side of said second link and being ofsubstantially uniform width, the ends of said slot at said intermediateportion being arcuate to define first seat portions for line contactwith said part-spherical portion, the inner sides of the bights of saidarms defining second seat portions for line contact with saidpart-spherical portion and said first and second seat portions beinglocated with respect to each other so as to capture and hold saidpart-spherical portion while said cylindrical portion is disposed withinsaid slot.
 2. A detachable link type of device as defined in claim 1wherein said arms are of substantially square cross section of sidedimension x, the radius of said spherical surface portion being R andthe radius of said cylinder being R-x where x is approximately equal toR(1-1/√2).
 3. A detachable link type device as defined in claim 1wherein each link is of a height substantially equal to the diameter ofsaid spherical surface portion.
 4. A detachable link type of devicewhich comprises:a first link having opposite end portions connected by afirst intermediate portion and a second link having opposite endportions connected by a second intermediate portion, one end portion ofthe first link having a spherical surface portion and the firstintermediate portion being in the form of a cylinder joined to said oneend portion to define a cylindrical, annular surface immediatelyadjacent said one end portion; one end portion of said second link beingadapted to receive and capture said one end portion of the first link insnap-fit relation, said one end portion of the second link including apair of generally U-shaped and parallel arms extending from said secondintermediate portion and which are spaced apart a distance less than thediameter of said spherical surface portion to define a slottherebetween, said slot being of a width substantially equal to thediameter of said cylinder and said arms being resiliently deformable tospread apart and pass said spherical surface portion therebetween andthen snap back toward said cylinder to capture said spherical surfaceportion of the first link, said spherical surface portion being providedwith a cylindrical stub upstanding therefrom and having an axis radialto said spherical surface portion and perpendicular to the axis of saidcylinder, said stub being of a diameter substantially equal to the widthof said slot and comprising means for coacting with said arms forpermitting relative rotation of said links about their longitudinal axesonly in forcible stepwise fashion.
 5. A detachable link type of deviceas defined in claim 4 wherein said spherical surface portion is providedwith a second upstanding cylindrical stub in diametrically opposedrelation to the stub first mentioned.
 6. A detachable link type ofdevice as defined in any one of claims 1, 4 or 5 wherein the other endportion of said first link is identical to said one end portion thereof.7. A detachable link type of device as defined in any one of claims 1, 4or 5 wherein the other end portion of said second link is identical tosaid one end portion thereof.
 8. A detachable link type of device asdefined in claim 6 wherein the other end portion of said second link isidentical to said one end portion thereof.
 9. A detachable link type ofdevice as defined in any one of claims 1, 4 or 5 wherein the other endportion of said first link is identical to said one end portion of thesecond link.
 10. A detachable link type of device as defined in any oneof claims 1, 4 or 5 wherein the other end portion of said second link isidentical to said one end portion of said first link.
 11. A detachablelink type of device which comprises:a first link having opposite endportions connected by a first intermediate portion and a second linkhaving opposite end portions connected by a second intermediate portion;one end portion of the first link having a spherical surface portion andthe first intermediate portion being in the form of a cylinder joined tosaid one end portion to define a cylindrical, annular surfaceimmediately adjacent said one end portion; and one end portion of saidsecond link being adapted to receive and capture said one end portion ofthe first link in snap-fit relation, said second intermediate portioncomprising a pair of parallel webs having inner surfaces spaced apart adistance less than the diameter of said spherical surface portion andone side edge of each web providing a seat for said spherical surfaceportion, said one end portion of the second link comprising a pair ofgenerally parallel branches extending from and in the plane of each weband adjacent the opposite ends thereof, the branches at one end of thetwo webs being joined by a first intermediate piece and the branches atthe opposite two ends of the webs being joined by a second intermediatepiece so that the joined branches define a slot extending from the oneside edge of one web to the one side edge of the other web, said slotbeing of substantially uniform width, said slot being of a widthsubstantially equal to the diameter of said cylinder and said branchesbeing resiliently deformable to allow said first and second intermediatepieces to spread apart to pass said spherical surface portiontherebetween and then snap back toward and substantially into engagementwith said cylinder, the ends of said slot at said webs being arcuate todefine first seat portions for line contact with said spherical surfaceportion, the inner sides of said intermediate pieces defining secondseat portions for line contact with said spherical surface portion, andsaid first and second seat portions being located with respect to eachother so as to capture and hold said spherical surface portion whilesaid cylinder is disposed within said slot.
 12. A detachable linkassembly comprising a first link having a penetrating end and a secondlink having a receiving end, said penetrating end being in the form of apart-spherical surface and having a cylindrical first intermediateportion joined thereto, said receiving end being in the form of a pairof U-shaped arms spaced apart substantially in accord with the diameterof said cylindrical portion to define a slot and having a secondintermediate portion joined thereto, each of said arms having two legportions joined by a bight portion and the free ends of the legs of onearm being joined to the corresponding free ends of the legs of the otherarm so that the arms are spreadable with respect to each other to passsaid spherical penetrating end and then to snap back to capture saidsaid penetrating end with said cylindrical intermediate portion beingreceived within said slot, said slot extending from said secondintermediate portion on one side of said second link to the secondintermediate portion on the other side of said second link and being ofsubstantially uniform width, the ends of said slot at said secondintermediate portion being arcuate to define first seat portions forline contact with said part-spherical portion, the inner sides of thebights of said arms defining second seat portions for line contact withsaid part-spherical portion and said first and second seat portionsbeing located with respect to each other so as to capture and hold saidpart-spherical portion while said cylindrical portion is disposed withinsaid slot.
 13. A detachable link assembly as defined in claim 12 whereinsaid part-spherical surface is entirely spherical except where it joinssaid cylindrical intermediate portion whereby said links are free topivot with respect to each other with said cylindrical portiontraversing said slot and also to rotate freely with respect to eachother about the axis of said cylindrical portion.
 14. A detachable linkassembly comprising a first link having a penetrating end and a secondlink having a receiving end, said penetrating end being in the form of apart-spherical surface and having a cylindrical first intermediateportion joined thereto, said receiving end being in the form of a pairof U-shaped arms spaced apart substantially in accord with the diameterof said cylindrical portion to define a slot and having a secondintermediate portion joined thereto, each of said arms having two legportions joined by a bight portion and the free ends of one arm beingjoined to the corresponding free ends of the legs of the other arm sothat the arms are spreadable with respect to each other to pass saidspherical penetrating end and then to snap back to capture said saidpenetrating end with said cylindrical intermediate portion beingreceived within said slot, said part-spherical surface being providedwith a cylindrical stub upstanding radially therefrom and having an axisperpendicular to the axis of said cylindrical portion, said stubcomprising means for coacting with said arms to permit relative rotationof said links with respect to each other about their longitudinal axesonly in forcible stepwise fashion.
 15. A detachable link assembly asdefined in claim 14 wherein said stub is disposed within said slot. 16.A detachable link assembly as defined claim 14 wherein saidpart-spherical surface is provided with a second cylindrical stubupstanding therefrom and diametrically opposed to the first mentionedstub.
 17. A detachable link assembly as defined in claim 16 wherein saidstubs are received in said slot to permit relative rotation between saidlinks only in forcible stepwise fashion as said cylindrical portiontraverses within said slot.
 18. A detachable link assembly whichcomprises at least a pair of separable links, one of said linksincluding penetrating means for insertion into a second link in eitherof two different relative orientations, the second link includingreceiving means for detachably capturing the penetrating means whileallowing free relative motion between said links when said penetratingmeans is captured therein in one of said relative orientations and forcoacting with structure on said penetrating means to permit onlyforcible stepwise relative motion therebetween when said penetratingmeans is captured therein in the other of said relative orientations.19. A detachable link assembly as defined in claim 18 wherein saidrelative motion is in a predetermined plane.
 20. A detachable linkassembly as defined in claim 18 including a plurality of said linksforming a chain, at least one pair of said links being joined in one ofsaid relative orientations and another pair of said links being joinedin the other of said relative orientations.
 21. A detachable linkassembly as defined in claim 18 wherein said penetrating means comprisesa part-spherical surface having a cylindrical portion joined thereto,said receiving means comprising a pair of spaced U-shaped arms in whichthe spacing between the arms is substantially the diameter of saidcylindrical portion.
 22. A detachable link assembly as defined in claim21 wherein each arm presents a pair of legs joined by a bight portion,the spacing between the legs of each arm being substantially equal tosaid diameter of said cylindrical portion, said arms being ofsubstantially square cross section of side dimention x, the radius ofsaid part-spherical surface being R and the radius of said cylindricalportion being R-x where x is approximately equal to R(1-1/√2).
 23. Adetachable link assembly as defined in claim 22 wherein the height andwidth of each link is approximately equal to the diameter of saidpart-spherical portion.
 24. A detachable link assembly which comprisesat least a pair of separable links, one of said links having apart-spherical penetrating end portion, a cylindrical first intermediateportion and a first opposite end portion, the other of said links havinga receiving end portion, a second intermediate portion and a secondopposite end portion, each of said links being of a length slightlygreater than twice the diameter of said part-spherical portion andhaving both a width and a height substantially equal to the diamter ofsaid part-spherical portion, said receiving end portion comprising apair of U-shaped arms having ends extending from said secondintermediate portion to define a slot therebetween and each of said armsincluding a bight, said arms being resiliently deformable so as tospread apart to pass said part-spherical portion and then snap back totrap said cylindrical portion therebetween, said slot being of a widthsubstantially equal to the diameter of said cylindrical portion and of alength sufficient to allow about 180 degrees of pivotal motion betweensaid links while said cylindrical portion traverses within said slot,said slot being of substantially uniform width, said second intermediateportion at the ends of said slot being arcuate and providing a firstseat against which said part-spherical portion engages with line contactand the bights of said arms providing a second seat against which saidpart-spherical portion engages with line contact so as to allow 360degrees of rotation between said links about the axis of saidcylindrical portion.
 25. A detachable link assembly which comprises atleast a pair of separable links, one of said links having apart-spherical penetrating end portion, a cylindrical first intermediateportion and a first opposite end position, the other of said linkshaving a receiving end portion, a second intermediate portion and asecond opposite end portion, each of said links being of a lengthslightly greater than twice the diameter of said part-spherical portionand having both a width and a height substantially equal to the diameterof said part-spherical portion, said receiving end portion comprising apair of U-shaped arms having ends extending from said secondintermediate portion to define a slot therebetween and each pair of armsbeing joined by a bight, said arms being resiliently deformable so as tospread apart to pass said part-spherical portion and then snap back totrap said cylindrical portion therebetween, said slot being of a widthsubstantially equal to the diameter of said cylindrical portion and of alength sufficient to allow about 180 degrees of pivotal motion betweensaid links while said cylindrical portion traverses within said slot,said second intermediate portion providing a first seat against whichsaid part-spherical portion engages and the bights of said armsproviding a second seat against which said part-spherical portionengages so as to allow 360 degrees of rotation between said links aboutthe axis of said cylindrical portion, said part-spherical portion beingprovided with an upstanding cylindrical stub having an axis radial tosaid part-spherical portion and perpendicular to the axis of saidcylindrical portion, said stub being of a diameter substantially that ofsaid cylindrical portion and comprising means for coacting with saidarms for requiring certain rotations between said links to be effectedin forcible stepwise fashion.